The seminiferous epithelium, the site of spermatogenesis, is populated by somatic Sertoli cells and developing spermatogenic cells. There is mounting evidence in mammals that these cells interact extensively and that these interactions are essential for the development of the male gamete. One important element in these interactions is the synthesis of particular Sertoli cell secretory proteins at specific stages of the cycle of the seminiferous epithelium. Such stage-specific changes in Sertoli cell function suggest that proteins which are secreted in a stage specific manner regulate particular events during spermatogenesis and that germ cells at particular stages of development stimulate these stage-specific Sertoli cell functions. We have recently isolated, generated antisera against and cloned the gene for a rat Sertoli cell secretory protein which we initially called Cyclic Protein-2 (CP-2). Using our reagents, we demonstrated that the mRNA levels and the synthesis of this protein both increased 30-fold from stage XII to stage VI of the cycle of the rat seminiferous epithelium. Immunocytochemical analysis of rat testes demonstrated that CP-2 is detected at stages V-VII in Sertoli cell cytoplasm adjacent to compacted spermatids. Sequence analysis of the cDNA for CP-2 revealed that this protein is, in fact, the proenzyme form of the protease, Cathepsin L. The central hypothesis of this application is that specific germ cells stimulate the transcription of the CP-2/cathepsin L gene by mature Sertoli cells and that this secreted protease binds to germ cells and facilitates their movement within the seminiferous tubule. We will test this hypothesis in four specific aims. First, we propose to analyze the cellular distribution, binding and uptake of CP-2/cathepsin L by Sertoli cells and germ cells. Second, we will study the effect of specific germ cells on steady-state levels of CP-2/cathepsin L mRNA in mature Sertoli cells. Third, we will analyze the transcription of the CP- 2/cathepsin L gene and stability of the transcript in mature Sertoli cells which are in contact with different types of germ cells. Fourth, we will identify the genetic elements which regulate expression of the CP- 2/cathepsin L gene in mature Sertoli cells.